1. Field of the Invention
This invention relates to a vibration-absorbing system for an automotive vehicle, and more particularly to a system designed for absorbing low-frequency vibration of the vehicle.
2. Description of the Prior Art
In general, low-frequency vibration in an automotive vehicle may result for a number of reasons such as, for example, combustion of the air/fuel mixture in the engine, fluctuation of engine rotational speed during engine idling operation, a heterogeneous distribution of a mass on the vehicle tire or a hardness on the tire periphery, an non-uniform configuration of the rotatable portion in a brake, etc. This vibration forcedly shakes the vehicle body and in turn passenger seats mounted on the vehicle body or the steering wheel shaft supported on the vehicle body. Such vibrations cause driver and passenger discomfort and annoyance.
A vehicle body usually has a resonance frequency in relation to the forced mechanical-vibration thereof. According to experiments for determining the resonance frequency of a normal passenger car body, the resonance frequency ranges approximately from 21 to 23 Hz. Experiments have been performed by measuring the amplitudes of vibrations of the vehicle body powered by a four-cylinder, four-cycle engine at varying engine rotational speeds. In this case, the engine serves as a frequency-variable vibration source for forcedly shaking the vehicle body with a frequency depending upon the rate of combustion of the air/fuel mixture in the engine.
The results of the experiments are shown in FIGS. 5 and 6. The amplitudes of vertical vibrations of the steering wheel supported on the vehicle body peak at engine speeds near 700 (RPM), as shown by the solid curve D of FIG. 5. The amplitudes of the vertical vibrations of the seat mounted on the vehicle body peak at engine speeds near 650 (RPM), as shown by the solid curve E of FIG. 6. Since the rate of combustion of the air/fuel mixture is twice the frequency of the engine crankshaft rotation in the four-cylinder, four-cycle engine, the resonance frequency of the vehicle body is estimated at a value ranging approximately from 21 to 23 Hz. Unfortunately, engine speeds of 650 to 700 (RPM) are encountered under engine idling operations. Thus, under idling conditions low-frequency vibrations are particularly troublesome.
A conventional methed of suppressing such a low-frequency vibration in an automotive vehicle is to enhance the hardness of the vehicle body. The method, however, necessitates reinforcement or increase in thickness of vehicle body members, making the vehicle heavier and increasing the number of steps of manufacturing or assembling the vehicle.